I'm looking into starting a keg setup. If I use a beer gun does it fill a bottle like a normal bottle, cap it and drink it whenever like a normal bottle? Or is it more like getting a bomber filled where you only have a day or two to drink it before its flat?

I use mine for beer and wine....the beer will retain it's carbonation indefintely. It's really no different than a counter pressure filler, all the counter pressure device does is prevent excess foaming by equalizing pressure....with the Beergun, you transfer using a lower pressure, about 5psi, this inhibits foaming.